Add fixture for CCD data and marker to set size
Add markers for scale and mean of distribution
Use fixture for ccd data instead of generating in each function
Refactor ccd data setup and addition of cosmic rays to test images

There were a number of smaller changes:
+ Cosmic ray flux increased to ensure all are caught given threshold
+ Change places where number of cosmic rays ways hard-wired to use number set at top of tests
PEP8 changes
Merge pull request #34 from mwcraig/ccddata-pytest-fixture

Add a pytest fixture to generate a test image